A film bulk acoustic resonator (FBAR) includes a stack of a bottom electrode, a piezoelectric thin film layer, and a top electrode. (The bottom electrode, the piezoelectric thin film layer, and the top electrode are collectively referred to herein as the “stack” and the “FBAR stack.”) When an electrical signal is applied to the top and bottom electrodes, the piezoelectric thin film layer converts the electrical energy of the signal into mechanical energy. An oscillating electrical signal applied to the piezoelectric thin film layer causes pressure and/or shear waves to propagate through the bulk of the FBAR stack. The waves in the stack are referred to as bulk acoustic waves. The bulk acoustic waves have their primary resonance in the stack at frequencies that are determinable from the thicknesses of the piezoelectric film and electrode layers.
The performance of a band-pass filter is improved by a structure that resonates at a desired frequency (e.g., the fundamental frequency) and impedes spurious resonances, such as unwanted minor resonances at frequencies other than (e.g., nearby) the fundamental frequency. Typically, FBARs operate primarily with longitudinal bulk acoustic waves, which propagate perpendicular to the plane of the piezoelectric thin film layer. The bulk waves reflect from the free surfaces at the top and the bottom of the stack, yielding sharply defined fundamental resonant frequencies, which is useful for constructing band-pass filters. The structure of an FBAR tends to result in a series of minor resonances with frequencies close to the fundamental resonant frequency, causing unwanted ripples (e.g., ripples that can be seen in an FBAR impedance over frequency graph). For example, the spurious resonances are caused by, e.g., standing lateral waves (plate waves) as a result of reflections from the edges (e.g., sides) of the FBAR stack and/or reflections from the edges of the cavity below the FBAR stack, resulting in ripples near the main resonant frequencies. These impedance ripples can affect the filter performance (e.g., loss over frequency) of an FBAR. Suppression of spurious resonances that cause impedance ripples, using the techniques described herein, can improve the filter performance of an FBAR.